As LSI circuits are increasing in density, the line width of circuits of semiconductor devices is becoming finer. To form a desired circuit pattern onto a semiconductor device, a method of reducing and transferring, by using a reduction-projection exposure apparatus, onto a wafer a highly precise original image pattern (mask, or reticle, in particular, when used in a stepper or a scanner) formed on a quartz is employed. The highly precise original image pattern is written by using an electron beam writing apparatus, in which a technology commonly known as electron beam lithography is used.
A writing apparatus which performs electron beam writing on a mask includes, for instance, a first aperture plate in which a rectangular opening is formed, and a second aperture plate in which a variable shaping opening is formed. An electron beam which has passed through an opening of the first aperture plate and has been shaped in a rectangular form is deflected by a deflector, passes through a variable shaping opening of the second aperture plate, and is shaped in desired shape and size. An electron beam which has passed through the second aperture plate is emitted to a sample placed on a movable stage.
When a variation in a sample surface in the height direction occurs due to bending or the like of the sample, the sample surface and an optimal focus are misaligned, and a pattern dimensional accuracy deteriorates. Therefore, a correction lens is provided which corrects a focal point deviation associated with a variation in the height direction of the sample surface, and the focal point variation is corrected during writing.
The optimal focus also varies with shot size. Therefore, it is preferable that a focus variation amount due to a shot size be corrected. The above-mentioned correcting lens may be used for correction of the focus variation amount due to a shot size. However, the focus variation amount associated with a variation in the height direction of the sample surface is greater than the focus variation amount due to a shot size. Therefore, the output of the DAC (digital to analog conversion) amplifier, which applies a voltage to the above-mentioned correcting lens, is greater than necessary, and a throughput is reduced due to a long waiting time until the output of the DAC amplifier is stabilized.